1. Field of the Invention
The present invention relates to a laser energy measuring unit and a laser machining apparatus having the laser energy measuring unit.
2. Related Art
Hitherto, when machining a blind hole (hereinafter referred to simply as a bore) that connects layers of a build-up type printed circuit board by the laser machining apparatus, an insulating layer formed by resin containing fillers such as fiberglass reinforced resin is removed by irradiating a laser through an etching window formed by removing an outer copper layer by etching in advance in a conformal mask method, or by directly irradiating a laser to the insulating layer having no outer copper layer in a direct method.
In this case, machining results vary when an output of the laser oscillator fluctuates. Then, in order to improve machining reliability, the laser to be supplied to the machining section is partly branched (1 to 2% of the laser to be supplied to the machining section) to measure energy of the branched laser and to machine while confirming that magnitude of the energy (referred to simply as “energy” hereinafter) of the laser is within a predetermined range.
FIG. 6 is a schematic section view of a prior art laser energy measuring unit along a measuring laser beam.
A condenser lens 17 and an energy measuring section 19 are disposed on an optical path of the measuring laser beam (called a “monitoring beam” hereinafter) 2b branched from the laser oscillated from the laser oscillator. The energy measuring section 19 is disposed so that a center of a sensor face of a sensor 23 substantially coincides with a focal point of the condenser lens 17.
Next, an operation of the laser energy measuring unit will be explained.
The monitoring beam 2b is condensed by the condenser lens 17 and enters the sensor 23. A calculating section 19a of the energy measuring section 19 compares an output of the sensor 23 with a value set in advance. When the energy of the monitoring beam 2b deviates out of the preset range, the energy measuring section 19a outputs an alarm signal to a control section 30 of the laser machining apparatus. Receiving the alarm signal from the energy measuring section 19, the control section 30 memorizes coordinates of a hole machined when it received the alarm signal and conducts a process specified in advance (sounds an alert buzzer or sometimes stops machining for example).
A diameter of a machining laser beam varies corresponding to a diameter of a bore to be machined. A diameter of the monitoring beam 2b entering the sensor 23 also changes, and the energy changes along with that. Therefore, it is necessary to use a sensor whose energy detecting range is as wide as the sensor 23. However, a sensor having a wide detecting range is expensive.
There has been proposed a technology of enabling even a sensor whose detecting range is narrow to measure the energy of laser by attenuating the energy of the monitoring beam 2b as disclosed in Japanese Patent Application Laid-open No. 2003-136267 for example. According to this technology, an attenuating member in which through holes of very small diameter are formed in mesh through a plain plate of a material that does not transmit the monitoring beam 2b is disposed on an incident side of the condenser lens 17 to attenuate the energy of the monitoring beam 2b. When the diameter of the monitoring beam 2b changes, the attenuating member can attenuate the energy corresponding to the diameter of the monitoring beam 2b. Accordingly, even the low cost sensor 23 can measure the energy of the monitoring beam 2b. 
However, an energy attenuation rate of the attenuating member described above is uniform. Therefore, when the attenuation rate is once set by deciding the diameter and density of the through holes disposed on the plate by arranging to a maximum diameter of the monitoring beam 2b, energy detecting precision drops when the diameter of the monitoring beam 2b is minimum, causing variations in the measured results.
Accordingly, an object of the invention is to provide a laser energy measuring unit whose laser energy measuring range is wide and whose measuring precision is high.
It is another object of the invention to provide a laser machining apparatus that stops machining so as not to degrade its machining precision when the magnitude of the energy of the laser measured by the laser energy measuring unit deviates out of a predetermined range.